1. Field of the Invention
The present invention relates generally to digital communication systems, and more particularly to a method that recognizes differences in characteristics among the information fields of digital. information that is protected by a channel code, and acts in recognition of these differences when transmission errors are found.
2. Description of the Related Art
In a digital communication system, digital information such as voice, data, image, or video, for example, passes from one location to another over a communication channel in the form of packets or frames of symbols such as binary digits (bits). When the communication channel is subject to a disturbance such as electrical noise, one or more bits crossing the channel may be altered, so that the information arriving at the far end of the channel is not received exactly as intended. Bits so altered are said to have been subject to transmission error, and the packet or portions thereof into which transmission errors fall are said to be flawed.
Errors are introduced by communication channels to a greater or lesser degree, dependent upon the physical makeup of the channels. At one extreme, a fiber-optic channel that is operating properly introduces transmission errors only rarely, because a fiber-optic channel is essentially invulnerable to the influence of its surroundings. At the other extreme, a radio link, even when designed properly and operating correctly, is highly subject to transmission errors caused by signal fading, atmospheric disturbances, intervening obstacles, limitations of radiated signal strength, interference caused by other radio activities, and a myriad of related phenomena.
To guard against transmission errors in a digital communication system, channel codes are used. These codes generate parity bits, a form of redundancy, that are included within the packet. In view of the added redundancy, a receiver can detect the presence of transmission errors with some degree of certainty, and often correct these errors.
However, the capability of a channel code to detect or correct transmission errors comes at a price: the parity bits require additional transmission bandwidth or impose longer transmission times, and therefore cause economic inefficiency. Further, the apparatus required to encode and decode is complex. In the world of miniature wireless communication terminals with extensive data communication features, the burden of the channel coding and decoding apparatus ultimately becomes onerous in proportion to its expense and in proportion to its need for space, heat dissipation, and battery power. So, despite the pressing need for channel coding in wireless systems, which are inevitably prone to transmission error, unlimited channel coding cannot be employedxe2x80x94rather, a practical balance must be struck between the capability of the channel code and the constraints on bandwidth, economics, and terminal size.
Further complicating the question of balance, transmission errors that fall into the header of a packet are often more troublesome than transmission errors that fall into the payload of a packet, because the header often carries information that is crucial to delivering the packet to the proper destination. In response to the need to distinguish between header errors and payload errors, the related art teaches the use of a first set of parity bits to protect the header and a second set of parity bits to protect the payload. Typically, the first parity bits come from a cyclic redundancy check (CRC) that is computed according to a relatively low-order generating polynomial, and the second parity bits come from a CRC computed according to a relatively high-order generating polynomial. For example, a four-bit CRC field may be included as the last field of the header, and a sixteen-bit CRC field may be included as the last field of the payload.
However, the above error control method is inherently inefficient. On the one hand, the redundancy bits devoted to protecting the header consume valuable transmission time or bandwidth, and the number of redundancy bits must therefore be limited. On the other hand, a limited number of redundancy bits have only a limited capability to protect crucial information carried by the header. Further, the receiver needs two separate CRC decoders, one for the header check and one for the payload check. The need for two separate decoders introduces unwanted complexity. Finally, the above error control method does not adapt to changes in the characteristics of the flawed information, as each of the two separate CRC decoders acts separately and blindly upon the bits under its protection. This method taught by the related art is therefore inflexible. For instance, the method requires a plurality of different packet structures to accommodate a spectrum of multimedia applications wherein the location of the crucial information within a packet does not necessarily follow the simple header-payload dichotomy, but rather changes from application to application and packet to packet.
Alternately, the related art teaches the use of complex channel codes that attempt to provide unequal protection of the bits within the confines of a single packet. Unfortunately, these codes have a number of shortcomings that limit their practical usefulness. In some instances, such codes may provide little difference between the maximum and minimum degree of protection of the bits under their protection. In other instances, such codes suffer greatly degraded performance when stretched to differentiate significantly between maximum and minimum. Moreover, were such a code required to adapt to the ever-changing demands of a multimedia system, wherein the location of crucial information within the packet may change from a first application to a second application, and from a first packet to a second packet even for a first application, the resulting encoder and decoder would be prohibitively complex, and therefore expensive, undesirable, and prone to consume excessive battery power when used in a portable device such as, for example, a miniature wireless communication terminal.
In view of these and other limitations of the related art, a need exists for a method of determining which subsets of digital information are flawed by transmission error and acting in recognition of differences in the natures of these subsets, where the method is: a) suitably flexible to meet the demands of multimedia traffic; b) strong when needed in its capability to protect against transmission errors; and c) efficient in. its use of battery power, transmission bandwidth, and processing resources so that it may be practically applied to wireless communications.
It is therefore an object of the present invention to provide a method of responding to transmission errors in information that flows across a digital communication system, wherein the method determines which subsets of the information are flawed, and acts in recognition of differences in the characteristics of these subsets.
It is therefore another object of the present invention to provide a method of processing digital information, wherein the method responds to the occurrence of transmission errors in a digital communication system, and the method is suitably flexible to meet the demands of multimedia traffic in which the location of crucial information within a packet may change from a first application to a second application and a first packet to a second packet.
It is yet another object of the present invention to provide an improved method of responding to transmission errors in a digital communication system, where the improved method is efficient in its use of electrical power, transmission bandwidth, and processing resources so that the improved method may be practically applied to wireless communications.
In a preferred embodiment of the invention, an incoming packet is decoded by computing the syndrome of the packet according to a channel code and examining the computed syndrome. When the syndrome is all-zero, the packet is passed up a communication protocol stack conventionally. When the syndrome is not all-zero, the decoding operation is continued by finding a coset leader associated with the syndrome. The coset leader is used to determine which fields of the packet are most likely flawed. Based on this determination, the packet is then rejected, corrected, or accepted without correction, according to the following method. When a field of critical importance is flawed, the packet is rejected. When the critical fields of the packet are intact but one or more of its fields of correctable importance are flawed, transmission errors in the correctable fields of the packet are corrected, and the packet is passed up the protocol stack. Otherwise, the packet is passed up the protocol stack uncorrected, as the critical and correctable fields of the packet are intact, but at least one field of tolerant importance is flawed.
In a preferred embodiment of the present invention, information is of critical importance (or xe2x80x9ccriticalxe2x80x9d) when it is inflexibly intolerant of error. Information is of correctable importance (or xe2x80x9ccorrectablexe2x80x9d) when it is tolerant enough of error to withstand the presence of an occasional error that survives the correction attempt of an error-correcting decoder. Information is of tolerant importance (or xe2x80x9ctolerantxe2x80x9d) if it may be readily used even though it contains a substantial number of transmission errors. Typically, one or more of the header fields of a packet are critical. Typically, the payload field of the packet is: a) critical when carrying data that is inflexibly intolerant of any risk of error such as, for example, financial data or key system-managment instructions, b) correctable when carrying, for example, image or data information for static display on a screen, and c) tolerant when carrying, for example, voice or video information for real-time playback. Typically, the parity field of the packet is correctable.
In accordance with a preferred embodiment, a method is provided for responding to transmission errors in digital information that is protected by a channel code and that is segmented into a plurality of fields such as a header field, a payload field, and a parity field, wherein the method comprises the steps of: decoding the digital information according to the channel code; identifying, responsive to the step of decoding, a flawed field of the plurality of fields; determining a characteristic of the flawed field; and processing the digital information according to the characteristic.
A preferred embodiment of the present invention addresses shortcomings of current methods for processing information in a mixed-media or a multimedia system, wherein some applications are more tolerant of transmission errors than others, and, even within a given packet, some transmission errors are more troublesome than others. This question of the relative importance of information flawed by transmission errors arises, for example, when a communication. system carries a mix of traffic that varies from key financial information such as credit card numbers, to ordinary alphanumeric text or image for display on a screen, to digitally encoded speech or video signals. Each of these applications demands a different approach to dealing with transmission errors.
When critical information is thought to be flawed, it should not be corrected; rather, it should be rejected in favor of the retransmission of new information, so as to minimize the risk of catastrophe. Under different circumstances, for example when text for ordinary screen display is thought to be flawed, transmission errors may be corrected by a channel code without undue risk, even though the would-be correction may itself be incorrect on rare occasion. Under yet different circumstances, for example when a packet carrying digitally encoded speech or video is flawed, its proper disposition depends on which fields of the packet have been affected. When transmission errors fall within the payload of the packet, the payload may sometimes be used without correction, as the presence of transmission errors in the payload may have little consequence beyond a passing degradation of fidelity. When, on the other hand, transmission errors fall within the header of the packet, the flawed packet might best be discarded, as the header often carries information that is crucial to delivering the packet to the proper destination. Alternately, the error might acceptably be corrected, depending on the nature of the specific field of the header that is flawed.
An advantage of the present invention is the capability to process information in a way that depends on the characteristics of a subset of the information, where that subset is flawed by transmission error.
Another advantage of the present invention is the adaptability to process information from a multimedia system effectively and economically when the information is flawed by transmission error.
Yet another advantage of the present invention is efficiency in the use of battery power, transmission bandwidth, and processing resources, so that the invention may be practically applied to wireless communications subject to frequent transmission error.